The weedeater engine WILL work...
But a standard Weed Eater 25cc engine revs up to typically 8-9000rpm, and puts out around 1hp
A Standard G260RC04 puts out around 2.9hp and revs up to 17-18000rpm, when ported by a GOOD tuner it puts out around 5.5 to 6.5hp with a tuned pipe and revs up to 20-22000rpm.
If you are really good at your porting work and replaces the standard weedeater carb with a better one, then the weedeater CAN be tuned to be competetive.
You also need to take into consideration that the RC style engines only uses 1 piston ring, reducing internal drag, and creating a better power potential.(Yes, Some Weedeaters also come with only a single piston ring)
A few of the weedeater engines I have seen, also has the challenge, that there is too little metal in the head, making it difficult to increase the size of the "flow channels", from the crankcase to the inlet ports.
The RC style engines also typically arrive with a lighter piston, than the Weedeaters, but that is just a "trimming" matter...
The RC Style engines are also available with a "full circle crank" that increases secondary compression and helps increasing power.
p.s. I am NOT a mechanic, I am best at replacing parts
p.p.s. Sorry for my bad English.